Shipyard welding typically involves high-strength low-alloy (HSLA) steels which are welded with high heat input. HSLA steels are characterized by low carbon contents which are generally less than 0.06 weight percent. The low-carbon high-strength nature of HSLA steels necessitates welding electrodes which form weld deposits with particular characteristics.
For example, conventional welding electrodes for HSLA steels generally have compositions as listed below in TABLE 1.
TABLE 1 ______________________________________ CONVENTIONAL WELDING ELECTRODE COMPOSITIONS FOR WELDING OF LOW ALLOY STEELS elemental AWS A5.28 MIL-E-23765/2D MIL-E-24355B composition 80S-D2 100S-1 140S-1 ______________________________________ carbon 0.07 to 0.12 up to 0.08 up to 0.12 manganese 1.60 to 2.10 1.25 to 1.80 1.50 to 2.00 silicon 0.50 to 0.80 0.20 to 0.55 0.30 to 0.50 nickel up to 0.15 1.40 to 2.10 1.95 to 3.10 chromium not specified up to 0.30 0.65 to 1.05 molybdenum 0.40 to 0.60 0.25 to 0.55 0.40 to 1.00 copper up to 0.50 up to 0.30 up to 0.15 titanium not specified up to 0.10 up to 0.04 aluminum not specified up to 0.10 up to 0.04 ______________________________________
(Compositions shown in TABLE 1 and elsewhere in this application, unless otherwise noted, refer to the weight percent of that element in the total electrode weight.)
In particular, TABLE 1 shows the composition ranges for three different welding electrodes used to weld high-strength, low-alloy steels. Those three welding electrodes correspond to three industry specifications: (1) American Welding Society (AWS) specification AWS A5.28, classification 80S-D2 for gas-shielded, metal-arc, low-alloy steel welding electrodes; (2) Military Specification MIL-E-23765-2D, classification 100S-1 for naval hull materials; and (3) Military Specification MIL-E-24355B, classification 140S-1 for naval hull materials.
Typically, welding electrodes with carbon contents greater than about 0.06 weight percent are specified for welding HSLA steels to promote the strength of the weld deposit. It has been found that the microstructure of weld deposits from conventional welding electrodes used for HSLA steels rely on martensite formation in order to achieve the required higher strength levels, and the strength of martensite is dependent on its carbon content. Higher carbon contents result in higher strength levels.
Moreover, the percentage of martensite formed, and thus the weld metal strength level is very dependent upon the weld metal cooling rate. Faster cooling rates promote martensite formation. Consequently weld metal cooling rates must be carefully controlled so that the amount of martensite formed is fairly constant, because variations in the amount of martensite will result in variations in the strength of the weld metal. For example, TABLE 2 shows the effect of heat input and cooling rate on the yield strength of conventional welding electrodes for welding HY-100 and HSLA-100 steels
TABLE 2 ______________________________________ Effect of Heat Input/Cooling Rate on Yield Strength for Conventional Welding Electrodes Cooling Yield Electrode Transfer Heat Input Rate Strength Type Mode (KJ/in) (.degree.F./sec) (ksi) ______________________________________ 120S-1 GMAW-Pulsed 30.1 58.3 128.0 120S-1 GMAW-Pulsed 78.7 6.4 95.0 120S-1 GMAW-Spray 30.3 75.4 132.0 120S-1 GMAW-Spray 79.7 5.8 98.5 ______________________________________
It should be appreciated, however, that the need to carefully control the weld metal cooling rate limits the useability of an electrode because it restricts the useful range of controllable parameters that affect the cooling rate--most particularly, welding heat input, interpass temperature and plate thickness.
In addition, the higher strength level of martensitic microstructures for conventional martensite-forming welding electrodes is offset by their greater sensitivity to hydrogen induced cracking and stress corrosion cracking. In order to minimize this tendency, the welding preheat and interpass temperatures must be maintained at sufficiently high levels so as to allow the hydrogen to diffuse out of the weld area. This precaution reduces the likelihood of hydrogen induced cracking.
Thus, a need exists for a family of welding electrodes which form weld deposits with the same strength as the martensitic microstructure weld deposits formed by conventional high-strength steel welding electrodes but which do not suffer from the disadvantages associated with those weld deposits.